Machines adapted to perform the electrical discharge machining (EDM) process with a continuous wire or a like electrode typically employ a pair of arm members individually extending from a machine column and provided at their respective free end portions with wire guide sections, generally in the form of rollers, to guide and stretch the wire electrode therebetween across a machining zone, the wire electrode being continuously fed from a supply reel provided on the machine column upstream of one of the guide sections and wound or collected on a take-up reel arranged on the machine column downstream of the other guide section. An electrical power supply is connected to the wire electrode and a workpiece which is disposed in juxtaposition therewith in the machining zone to effect a series of electrical discharges between the traveling wire electrode and the workpiece across a machining gap flushed with a liquid dielectric, e.g., distilled water, to remove material from the workpiece.
Means for feeding and collecting the wire electrode and transporting or advancing it through the machining zone generally includes drive means disposed in a path between the wire take-up reel and one guide section and brake means disposed in a path between the other guide section and the wire supply reel. The wire drive means typically comprise a drive roller continuously rotated by a motor and an idler roller rotated in frictional contact with the drive roller holding the wire electrode therebetween while the wire brake means may comprise a similar roller arrangement driven by a motor to impart to the wire electrode continuously fed by the drive rollers a brake force sufficient for the latter to be stretched straight under tension and thus tightly between the wire guide sections. The necessary brake force or tension is obtained, for instance, by adjusting the rate of rotation of the feed drive roller several percent greater than that of the brake drive roller.
The guide sections are required not only to position the wire electrode precisely in a cutting relationship with the workpiece in the machining zone but also to serve to change the wire advance direction from the wire supply side to the machining zone and from the latter to the wire take-up side or thus to guide the wire electrode from one direction to that substantially normal thereto since the arm members respectively carrying the guide sections individually extend from the column which is vertically mounted on a machine base which carries a machining table supporting the workpiece and the wire supply reel and the wire take-up reel are both arranged on or in the machine column. As a result, considerable bending forces develop of the arm members so that the latter tend to deflect inwardly or toward the machining region.
The wire electrode is generally very thin and its change in frictional stresses at drive and brake roller portions results in change in tension applied on the wire electrode and hence in change in deflection of the arms which guide the wire electrode at their free end portions. The deflection of the arms tends also to change due to variation in other external forces and irregularity of temperature distribution over the arms.
Change in the deflection of the arms is disadvantageous since it causes deviation of the wire electrode from the preset machining position relative to the workpiece and accordingly results in the inability to perform cutting in the latter along a prescribed path and hence lack of machining accuracy. Furthermore, the fluctuations of the beding of the guide arm members can cause short-circuiting between the wire electrode and the workpiece.